a) Field of the Invention
The invention is directed to an arrangement for generating extreme ultraviolet radiation by means of an electrically operated gas discharge with a discharge chamber which has a discharge area for a gas discharge for forming a plasma that emits the radiation, a first disk-shaped electrode and a second disk-shaped electrode, at least one of which electrodes is mounted so as to be rotatable, an edge area to be covered by a molten metal, an energy beam source for providing a pre-ionization beam, and a discharge circuit connected to the electrodes for generating high-voltage pulses.
b) Description of the Related Art
Studies carried out on a large number of electrode shapes for gas discharge sources such as Z-pinch electrodes, hollow-cathode electrodes, plasma focus electrodes or star pinch electrodes have shown that the lifetime of electrodes formed in this way is not sufficient for EUV projection lithography.
However, rotating electrodes, as they are called, have turned out to be a very promising solution for appreciably prolonging the life of gas discharge sources. One advantage is improved cooling of these electrodes, which are disk-shaped in particular. Further, shortening of the lifetime due to inevitable electrode erosion can be eliminated by continuously renewing the electrode surface.
A previously known device according to WO 2005/025280 A2 uses rotating electrodes that dip into a vessel containing molten metal, e.g., tin. The metal that is applied to the electrode surface is evaporated by laser radiation, whereupon the vapor is ignited by a gas discharge to form a plasma.
This technique is disadvantageous especially in that a desired layer thickness of the applied material can be adjusted only with difficulty. Further, on the one hand, upward of a certain rotational speed, spatter occurs and material exits from the bath when the disk-shaped electrodes are partially immersed in the molten metal. On the other hand, when the rotational speed is too low, unconsumed portions of electrodes are too slowly brought into the discharge area and cause instability in the plasma generation. This problem is particularly severe when applications require repetition rates of several kilohertz.
It would be desirable to adjust a distance between two areas on the electrode which serve successively as discharge zones so that this distance is greater than the radius of the area on the electrode surface serving as the discharge zone.